Just to cover some more possibilities:
<?php
$var1 = "foo";
$var2 = "bar";
echo $var1 . $var2 . "<br>\n";
// ...or...
echo $var1, $var2, "<br>\n"; // only with echo, not with other string literals
// ...or...
echo "$var1$var2<br>\n";
// ...or...
printf("%s%s<br>\n", $var1, $var2);
// ...or...
echo $var1;
echo $var2;
echo "<br>\n";
// ...or...
echo "{$var1}{$var2}<br>\n";
// ...or...
echo <<<EOD
$var1$var2<br>
EOD;
?>